The systems Sr6Ta2O11-H2O and Sr5.92Ta2.08O11.12-H2O were investigated. At high temperatures, both compounds are normal proton conductors but at lower temperatures show the formation of hydrate phases (here labelled p-phases). The latter are deleterious if the above compounds were to be used in fuel cells, sensors and pumping devices. The phase diagrams of the 2 systems were determined using thermo-ravimetry. The solubilities of water vapor in the a-phases were measured. Considering 2-phase mixtures of a and, the plateau vapor pressures were determined as well as the pertinent thermodynamic parameters. The ot alpha ->alpha+beta solvus was also investigated. The present study complements recent work [I. Animitsa, A. Nieman, A. Sharafutdinov, S. Nochrin, Solid State Ionics 136-137 (2000) 265; I. Animitsa, A. Nieman, S. Titova.. N. Kochetova, E. Isaeva, A. Sharafutdinov, N. Timofeeva, Ph. Colomban, Solid State Ionics 156 (2003) 95]. (c) 2005 Elsevier B.V. All rights reserved.